1. Field of the Invention
This invention relates generally to the prevention of bleeding of organic material onto a metal substrate and, more particularly, to preventing bleeding of adhesive or components thereof onto a noble metal substrate. In even more particular aspects, this invention relates to the prevention of the bleeding of any components of a die attach adhesive onto noble metal circuitry on a dielectric substrate and, specifically, onto gold wire bond surfaces on a chip carrier or other noble metal circuitry on an I/C chip mounting substrate.
2. Background Information
In the mounting of integrated circuit (I/C) chips on substrates having electrical connections, such as wire bond pads thereon, one technique of mounting utilizes a two-part epoxy adhesive to bond the chip either to gold plating on a planar surface of a substrate, or onto copper or some dielectric material in a cavity formed in an electrical substrate, which substrate also has electrical contacts or wire bond surfaces on the surface thereof. One particular problem encountered when using such bond techniques is the tendency of the components of the epoxy adhesive to "bleed" and spread onto the wire bond surfaces, such as the wire bond pads or other noble metal surfaces, such as ground rings or voltage rings, and adhere thereto. This "bleed" causes the surface of the bond pads or other metal surfaces to be unreceptive to the bonding of the wire bonds necessary to electrically connect the wire bond surfaces to the contacts on the I/C chip. This, of course, results in unsatisfactory electrical contacts and, thus, contributes to an unsatisfactory chip package.
One prior art technique for solving this problem is to treat the part with a CF.sub.4 plasma. The plasma reacts with the organic portion of the substrate, and the fluorine containing moieties settle on the gold surface of the wire bond pads and other gold surfaces and make them less susceptible to being wetted by bleed from the adhesive. However, this is not a completely satisfactory treatment in all instances in that the fluorine containing moieties are not strongly bonded to the gold but are only lightly adhered thereto. Therefore, they can be easily removed with organic solvents during processing before attachment of the I/C chip takes place, but not easily removed with aqueous systems during processing. Moreover, the CF.sub.4 plasma tends, in many cases, to adversely affect the dielectric material for subsequent application of encapsulating material.
It is, therefore, an object of the present invention to provide a method and structure for substantially reducing or eliminating the wettability of metal surfaces, particularly noble metal surfaces, and specifically gold or other noble metal surfaces, to organic materials, particularly any components of an adhesive, such as an epoxy used for attaching an I/C chip to a substrate, and which I/C chip is subsequently bonded thereto.